This invention relates to the structure of an original table for reading images to be used when the images are read with a linear sensor (one dimensional sensor) comprising a photo-electronic conversion element such devices as a CCD (charge coupled device).
Image reading devices which read originals with linear sensors have widely been utilized. As FIG. 1 schematically shows, an original 1 on an original table is moved in a direction N at a predetermined timing, and at the same time the images are being read in the unit of a linear region 1A with a linear sensor 10 comprising CCD or MOS (metal oxide semiconductor) via a fixed lens system 2. The one line of data is converted into electric signals, and processed for images. The images are thus read by scanning the whole original 1 while the original 1 is being transported in the direction N.
FIG. 2 shows a positional relation of the linear sensor 10 in cross section wherein the original 1 is placed on a transparent glass plate 3 constituting an original table, and is covered with another transparent glass plate 4. The original 1 is illuminated with light from a light source 5 such as a fluorescent lamp, and the light transmitted through the original 1 of light transparent type such as a sheet of negative film enters the linear sensor 10 via the lens system 2. As shown in FIG. 3, the glass plate 3 is provided with an original placement region 32 for mounting the original 1 and with trimming lines 31 at a predetermined interval on both sides and at the top of the glass plate 3 to be used as the reference in registering the original 1 precisely on the original table. The image reading region 1A of the linear sensor 10 is wider than the original placement region 32 to give sufficient margins so that the linear sensor 10 can read the regions inside the trimming lines 31 on both sides. The linear sensor 10 has an effective output region 11 at the center thereof and dark-time output regions 12 and 13 comprising several pixels on both sides thereof which are respectively structured with light-blocking members coated over light receiving surfaces. The outputs from the dark-time output regions 12 and 13 are signals outputted when the light is blocked corresponding to image signals read at the effective output regions 11 of the linear sensor 10 to provide reference signals and to substract the reference signals of the dark-time level from the image signals of the effective output region 11 to thereby output image signals corrected with the dark-time level.
In the conventional image reading method with such a linear sensor 10, the image reading region 1A of the linear sensor 10 is wider than the region 32 of the glass plate 3 as shown in FIG. 3, and the lens system 2 is adjusted as to position the dark-time output regions 12 and 13 at locations falling outside the placement region 32, or on the trimming lines 31. When the original table is moved in the direction N for scanning of the image reading, the dark-time output regions 12 and 13 of the linear sensor 10 unavoidably detect the trimming lines 31 outside the imaged region which do not transmit the light in the case shown in FIG. 2 while they detect the transparent regions of the glass plate 3 where the trimming lines 31 are not provided on the light transmitting regions in other cases. The dark-time output regions 12 and 13 pass the light transmitting region and non-transmitting region alternately to thereby inconveniently cause minor fluctuations in the output level. Processing of image signals read by the effective output region 11 is thus affected disadvantageously to thereby deteriorate the image quality. This is attributable to the fact that as the light blocking members coated on the light receiving surfaces of the dark-time output regions 12 and 13 have transmissivity of 1% or less, the changes in input light causes minute fluctuations in the output level.
If the glass plate 3 is not coated with the trimming lines 31, dust and scars on the glass plate tend to damage the surfaces and cause similar problems in the output from the dark-time output regions 12 and 13 as the input light changes.
Moreover, in the prior art, as the signals from the linear sensor 10 are digitalized by an A/D converter for image processing, it is necessary to measure maximum and minimum light intensities inputted in the linear sensor 10 in advance for calibrating the circuit system for signal processing. This is because if the maximum and minimum input light intensities are correctly known in advance, the system can precisely and securely read the most and least lit parts of any original.
The maximum input light intensity in the prior art is given as the light transmitting through the original table with the light source 5 turned on while the minimum input light is given as the light with the light source 5 turned off when there is no original 1 on the table. However, the operation to turn the light source 5 on and off is quite cumbersome, and the life of the light source 5 presents another problem. It also takes a long time for calibrating the circuit system.